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Abstract:

An enclosed, flat covered leadless pressure sensor assembly suitable for
extreme environment operation including dynamic, ultra-high temperature
heating, light and heat flash, and high-speed, flow-related environments.
The pressure sensor assembly comprises a substrate comprising a
micromachined sensing diaphragm defined on a first side. A cover is
attached to the first side of the substrate such that it covers at least
the sensing diaphragm. The top surface of the cover is substantially
flat, thereby promoting uniformity in the distribution of stress and
thermal effects across a top surface of the cover.

Claims:

1. A leadless pressure sensor assembly, comprising: a substrate; a boss
region and a membrane region defined on a first side of the substrate,
wherein the boss region and the membrane region collectively form a
pressure sensing diaphragm; and a cover attached to the first side of the
substrate such that it covers at least the diaphragm, wherein the cover
has a substantially flat top surface.

2. The pressure sensor assembly of claim 1, further comprising a sensing
element attached to a second side of the substrate.

4. The pressure sensor assembly of claim 2, further comprising a contact
glass cover attached to the second side of the substrate such that it
covers and hermetically seals at least the sensing element.

5. The pressure sensor assembly of claim 1, wherein the cover is of
uniform thickness.

6. The pressure sensor assembly of claim 1, wherein a bottom surface of
the cover is micromachined.

7. The pressure sensor assembly of claim 1, wherein the cover is
monolithically attached to the substrate.

8. The pressure sensor assembly of claim 1, wherein the cover and the
substrate are of substantially identical compositions.

9. The pressure sensor assembly of claim 1, wherein the cover
substantially covers the first side of the substrate.

10. The pressure sensor assembly of claim 1, wherein the cover completely
covers the first side of the substrate.

11. The pressure sensor assembly of claim 1, further comprising an
adhesive layer disposed between the substrate and the cover.

12. The pressure sensor assembly of claim 11, wherein the adhesive layer
is made of Pyrex® glass.

13. The pressure sensor assembly of claim 1, wherein the cover provides
stability to the pressure sensor assembly by strengthening the boss
region.

14. The pressure sensor assembly of claim 1, wherein the substrate is
made from a high temperature semiconductor material.

15. The pressure sensor assembly of claim 14, wherein the high
temperature semiconductor material is silicon or silicon carbide.

16. The pressure sensor assembly of claim 1, wherein the cover is made
from a high temperature semiconductor material.

17. The pressure sensor assembly of claim 16, wherein the high
temperature semiconductor materials is silicon or silicon carbide.

18. A leadless pressure sensor assembly, comprising: a substrate; a boss
region and a membrane region defined on a first side of the substrate,
wherein the boss region and the membrane region collectively form a
pressure sensing diaphragm; a sensing element mounted to a second side of
the substrate; a cover having a substantially flat surface attached to
the first side of the substrate such that it covers at least the
diaphragm, wherein the cover is of a uniform thickness; and an adhesive
layer disposed between the substrate and the cover.

19. The pressure sensor assembly of claim 18, further comprising a
contact glass cover attached to the second side of the substrate such
that it covers and hermetically seals at least the sensing element.

20. The pressure sensor assembly of claim 18, wherein the adhesive layer
is made of Pyrex® glass.

21. The pressure sensor assembly of claim 18, wherein the cover
substantially covers the first side of the substrate

22. The pressure sensor assembly of claim 18, wherein the cover
completely covers the first side of the substrate.

23. The pressure sensor assembly of claim 18, wherein the cover provides
stability to the pressure sensor assembly by strengthening the boss
region.

24. A method of making a substantially enclosed, flat covered leadless
pressure sensor assembly, comprising: micromachining a first side of a
substrate to define a boss region and a membrane region that collectively
form a pressure sensing diaphragm; and attaching a cover having a
substantially flat surface to the first side of the substrate such that
the cover covers at least the diaphragm.

25. The method of claim 24, further comprising disposing an adhesive
layer between the substrate and the cover.

26. The method of claim 24, further comprising mounting a sensing element
to a second side of the substrate.

27. The method of claim 26, further comprising hermetically sealing at
least the sensing element with a contact glass cover.

Description:

BACKGROUND

[0001] Many pressure sensor assemblies of the prior art are improperly
equipped to withstand extreme environments, such as harsh temperature
environments, high vibration environments, and conductive and corrosive
media environments.

[0002] Traditional wire bonded pressure sensors cannot be exposed to any
of these environments without damage. As another example, sensors that
utilize oil-filled technology, can be exposed to conductive and corrosive
environments, however the operable temperature range of the sensor is
significantly limited because of the presence of oil. As yet another
example, leadless sensors, wherein the sensing elements are mounted
upside down onto appropriately designed headers such that only the
backside of the sensing element is exposed to the pressure media, are
suitable for device operation in most extreme environments. In this
particular embodiment, illustrated in FIG. 1, however, when exposed to a
pressure media with high-dynamic thermal and flow conditions, the exposed
micromachined diaphragm can experience uneven heating, and other
flow-related stresses, because of the uneven shape of the exposed
surface. Therefore, in combustion measuring and hypersonic measuring
applications, the error associated with dynamic heat and flow related
phenomenon becomes significant and limits the accuracy of traditional
leadless sensors.

[0003] Because of the limitations presented in the above-mentioned prior
art embodiments, there is a need for a pressure sensor assembly suitable
for operation in extreme environments, including: (1) dynamic, ultra-high
temperature heating environments, (2) light and heat flash environments,
and (3) high-speed, flow-related environments.

BRIEF SUMMARY

[0004] The various embodiments of the present invention provide an
enclosed, flat covered leadless pressure sensor assembly suitable for
operation in extreme environments, such as dynamic, ultra-high
temperature heating, light and heat flash, and high-speed, flow-related
environments. The pressure sensor assembly of the present invention
comprises a substrate having a boss region and a membrane region defined
on a first side, wherein the boss region and the membrane region
collectively form a pressure sensing diaphragm. The pressure sensor
assembly further comprises a cover that is attached to the first side of
the substrate such that it covers at least the diaphragm. The cover has a
uniform flat top surface and promotes a uniformly applied pressure. An
adhesive layer may be disposed between the substrate and the cover to
enhance the bond therebetween.

[0005] Embodiments of the pressure sensor assembly may further comprise a
sensing element attached to a second side of the substrate, wherein the
sensing element is adapted to output a signal substantially indicative of
the applied pressure. In some embodiments, the pressure sensor assembly
may further comprise a contact glass cover attached to the second side of
the substrate such that it covers and hermetically seals at least the
sensing element.

[0009] FIGS. 4A and 4B illustrate a second embodiment of a flat covered
leadless pressure sensor assembly in accordance with exemplary
embodiments of the present invention using a shaped cover.

DETAILED DESCRIPTION

[0010] Although preferred embodiments of the invention are explained in
detail, it is to be understood that other embodiments are contemplated.
Accordingly, it is not intended that the invention is limited in its
scope to the details of construction and arrangement of components set
forth in the following description or illustrated in the drawings. The
invention is capable of other embodiments and of being practiced or
carried out in various ways. Also, in describing the preferred
embodiments, specific terminology will be resorted to for the sake of
clarity.

[0011] It must also be noted that, as used in the specification and the
appended claims, the singular forms "a," "an," and "the" include plural
referents unless the context clearly dictates otherwise.

[0012] Also, in describing the preferred embodiments, terminology will be
resorted to for the sake of clarity. It is intended that each term
contemplates its broadest meaning as understood by those skilled in the
art and includes all technical equivalents which operate in a similar
manner to accomplish a similar purpose.

[0013] Ranges may be expressed herein as from "about" or "approximately"
one particular value and/or to "about" or "approximately" another
particular value. When such a range is expressed, another embodiment
includes from the one particular value and/or to the other particular
value.

[0014] By "comprising" or "containing" or "including" is meant that at
least the named compound, element, particle, or method step is present in
the composition or article or method, but does not exclude the presence
of other compounds, materials, particles, method steps, even if the other
such compounds, material, particles, method steps have the same function
as what is named.

[0015] It is also to be understood that the mention of one or more method
steps does not preclude the presence of additional method steps or
intervening method steps between those steps expressly identified.
Similarly, it is also to be understood that the mention of one or more
components in a device or system does not preclude the presence of
additional components or intervening components between those components
expressly identified.

[0016] Referring now to the drawings, in which like numerals represent
like elements, exemplary embodiments of the present invention are herein
described. It is to be understood that the figures and descriptions of
the present invention have been simplified to illustrate elements that
are relevant for a clear understanding of the present invention, while
eliminating, for purposes of clarity, many other elements found in
typical pressure sensor assemblies and methods of making and using the
same. Those of ordinary skill in the art will recognize that other
elements are desirable and/or required in order to implement the present
invention. However, because such elements are well known in the art, and
because they do not facilitate a better understanding of the present
invention, a discussion of such elements is not provided herein.

[0017] Exemplary embodiments of the present invention provide a
substantially enclosed, flat covered, leadless pressure sensor assembly
suitable for operation in extreme environments such as dynamic,
ultra-high temperature heating, light and heat flash, and high-speed,
flow-related environments. The various embodiments of the pressure sensor
assembly of the present invention generally comprise a substrate having a
micromachined diaphragm defined on a first side and a sensing element
attached to a second side. A cover is attached to the first side of the
substrate, such that it substantially encloses the sensing diaphragm and
the substrate. The cover has a uniform flat top surface, which enables an
applied pressure and temperature to be uniformly distributed across the
cover. The pressure sensor assembly of the present invention may be
adjusted to a desired pressure range, therefore enabling high magnitude
pressure readings and highly accurate and stress isolated pressure
measurements in extreme environments.

[0018] An exemplary embodiment of the pressure sensor assembly 100 of the
present invention is illustrated in FIGS. 2A and 2B. The pressure sensor
assembly 100 comprises a substrate 105 having a first side 110 and a
second side 115. The substrate 105 may be made of silicon because silicon
has high thermal conductivity and high temperature tolerance properties.
The substrate 105 may be of various geometries and dimensions. One
skilled in the art will appreciate that these properties are dependent on
the type of environment and desired pressure range in which the pressure
sensor assembly is designed to operate. A diaphragm 120, which comprises
a boss region 155 and a membrane region 160, is micromachined on the
first side 110 of the substrate 105, and is configured to deflect upon
receiving an applied pressure. A series of sensing elements 125 are
mounted on the second side 115 of the substrate 105 such that it is
aligned with the diaphragm 120. In an exemplary embodiment, the sensing
elements 125 are piezoresistive sensing elements comprising four
piezoresistors, and outputs a signal substantially indicative of the
applied pressure. The sensing elements are mounted of the back side of
the substrate to isolate them from the pressure media.

[0019] As also illustrated in FIGS. 2A and 2B, a cover 130 is attached to
the first side 110 of the substrate 105. In an exemplary embodiment, the
cover 130 is made of silicon and is monolithically attached to the
substrate 105 during substrate fabrication. The cover 130 encloses the
diaphragm 120 and substantially, if not completely, encloses the
substrate 105. The cover 130 is of a uniform thickness, and therefore has
a substantially flat top surface 135, which promotes uniformity in the
distribution of stress and thermal effects across a top surface 135 of
the cover 130 that are commonly associated with dynamic heating and other
flow-related perturbations, distinguishing the pressure sensor assembly
of the present invention from the prior art pressure sensor assembly
illustrated in FIGS. 1A and 1B, as the uncovered diaphragm in the prior
art assembly causes uneven thermo-acoustic distributions across the
surface.

[0020] The cover 130 may be of various thicknesses, however, as stated
above, the cover 130 is of a uniform thickness throughout. One skilled in
the art will appreciate that the thickness of the cover 130 may be
adjusted to suit a particular pressure range and operating environment.
One skilled in the art will also appreciate that the thickness of the
diaphragm 120 may also be adjusted to suit a desired pressure range and
may be complementary to the thickness of the cover 130. The length,
width, and overall geometry of the cover 130 can be of many dimensions
and shapes, respectively. In an exemplary embodiment, however, the
length, width, and geometry of the cover 130 corresponds to the length,
width, and geometry of the substrate 105, such that the diaphragm 120 and
the substrate 105 are substantially, if not fully, covered by the cover
130. Accordingly, the diaphragm 120 and the substrate 105 are effectively
shielded from extreme environments.

[0021] In some embodiments, the top surface 135 of the cover 130 may be
coated with various combinations of inert, high temperature metal thin
films, for example but not limited to, gold and platinum. The presence of
these highly reflective metal films enables further protection to flash
and heat without introducing any performance-related errors to the
diaphragm 120.

[0022] In some embodiments, an "adhesive" layer 140 may be disposed
between the substrate 105 and the cover 130 to strengthen the bond
therebetween. The adhesive layer 140 provides a "glue line" between the
substrate 105 and the cover 130, via an electrostatic attachment
mechanism technique, and provides thermal insulation between the cover
130 and the diaphragm 120. Although some exemplary embodiments of the
present invention comprise an adhesive layer 140, it shall be understood
that the pressure sensor assembly 100 may or may not comprise an adhesive
layer 140. The adhesive layer 140 may be made from many materials, for
example but not limited to, Pyrex® glass.

[0023] A contact glass cover 145 may be attached to the second side 115 of
the substrate 105, such that it hermetically seals the sensing element
125 from extreme environments. In some embodiments, a glass frit filler
may be used to further secure the contact glass cover 145 to the second
side 115 of the silicon substrate 105. Also, it shall be understood that
the shape of the contact glass cover 140 corresponds to the shape of the
substrate 105, and thus can be of many geometries. The contact glass
completely covers the sensing elements 125 sealing them in a hermetic
cavity isolated from the pressure media. Because the sensing element 125
is effectively shielded from the external environment, high temperatures
and extreme environments will not interfere with its accuracy. As also
illustrated, the contact glass cover 145 may define two or more apertures
150 that are filled with a conductive metal-glass frit, which enables
electrical communication between the sensing element 125 and
corresponding output devices.

[0024] Standard leadless high pressure (e.g. 1000 PSI and higher) sensor
assemblies, such as the one illustrated in FIGS. 1A and 1B, exhibit
non-linearity due to imperfect stress gradients. In these pressure sensor
assemblies, as the diaphragm thickens in comparison to the thickness of
the bossed area, the boss region begins to have less effect on the stress
gradients, which means there is less stress concentration in the
piezoresistors. An unexpected advantage of the pressure sensor assembly
100 of the present invention, however, is that it stiffens the boss
region 155 to a greater extent as compared to the rest of the diaphragm
120. Specifically, the cover 130 adds stiffness to the entire diaphragm,
making it deflect less at the same thickness, as the cover 130 is clamped
directly onto the boss region 155.

[0025] FIGS. 3A and 3B graphically illustrate the stress concentrations of
the prior art pressure sensor assembly and the pressure sensor assembly
100 of the present invention, respectively. As illustrated in FIG. 3B,
the pressure sensor assembly 100 of the present invention has sharper
stress gradients and a better defined stress concentration, which allows
for more precise placement of the piezoresistors. This consequently leads
to an even distribution of stress across the four piezoresistors, which
provides a more linear device. The cover 130 increases stiffness,
improves thermal isolation, and promotes uniform stress across the
sensing element 125. These properties enable accurate pressure
measurements in combustion engine applications and other applications
where extreme thermoacoustic and flow events are experienced.

[0026] A second exemplary embodiment of the pressure sensor assembly 400
of the present invention is illustrated in FIGS. 4A and 4B. This
embodiment is similar to the embodiment illustrated in FIG. 2, however
the cover 130 has been replaced by a micromachined cover 430. The top
surface 435 of the cover 430 remains substantially flat and uniform to
present an even surface for pressure and temperature affects, while the
bottom surface 440 of the cover may be micromachined by one or more of
the standard micromachining techniques well known to those skilled in the
art. This micromachining allows for even tighter refining of the stress
concentrations in the piezoresistive sensing elements 425. The shaping
also allows for better control of heat transfer between the cover and
sensor creating a more uniform temperature profile in the sensing
elements 425. The shape of the cover 430 may be the same as the shape of
the diaphragm but it may also be different depending on the desired
stress and temperature profiles.

[0027] One skilled in the art will appreciate that materials other than
silicon may be used to make pressure sensors. Other high temperature
semiconductor materials, for example, silicon carbide may be used for
many of the pressure sensor assembly components to make a robust,
extremely high temperature sensor. For example, making the cover from
silicon carbide increases the durability of the sensor, allowing for long
term survivability in extremely harsh environments. As another example,
making the substrate layer and piezoresistors from silicon carbide allows
for a much higher temperature sensor. It is also possible to make the
contact layer from silicon carbide, which allows for a more robust
sensor.

[0028] Numerous characteristics and advantages have been set forth in the
foregoing description, together with details of structure and function.
While the invention has been disclosed in several forms, it will be
apparent to those skilled in the art that many modifications, additions,
and deletions, especially in matters of shape, size, and arrangement of
parts, can be made therein without departing from the spirit and scope of
the invention and its equivalents as set forth in the following claims.
Therefore, other modifications or embodiments as may be suggested by the
teachings herein are particularly reserved as they fall within the
breadth and scope of the claims here appended.